See Figures 1, 2, 3 and 4
The C-4 system is used on 1980 California vehicles having the 305 engine. The "system" was introduced because of increasing governmental pressures to meet more stringent fuel economy and emissions requirements.
The main components of the C-4 system and brief descriptions of each item follow.
Electronic Control Module (ECM)
The ECM is the actual computer "brain" which monitors and controls the C-4 system.
The PROM (Programmable Read-Only Memory) unit "tailors" the functions of the ECM to a particular vehicle. In the case of the 1980 Corvette, only one PROM unit is serviced because only one engine/transmission combination was available in Corvettes sold in California. In later years, many different PROM units are used for the same model, according to vehicle weight, drivetrain combinations, and emission requirements of where the vehicle is sold (e.g.-Federal/49 state, California, Canada, etc.).
The oxygen sensor constantly monitors the amount of oxygen in the engine exhaust. The sensor reads a high oxygen content as a lean mixture; low oxygen content as a rich mixture. The data from the oxygen sensor is processed by the ECM, causing the ECM to vary other system functions accordingly. A "sensor" flag on the instrument panel indicates when the oxygen sensor must be replaced.
To reset the "Emission" flag on the speedometer face, remove the instrument cluster lens. Insert a long, pointed instrument diagonally into the detents on the upper left side of the "Emission" wheel. Rotate the wheel downward until an alignment mark is visible in the left side of the odometer window.
Engine Coolant Temperature (ECT) Sensor
The ECT sensor monitors and signals the ECM concerning the engine coolant temperature. The sensor is mounted on the intake manifold and modifies the ECM output signal to compensate for a cold engine condition which improves the cold driveability of the engine.
Vacuum Control Switches
Various vacuum switches are used to monitor engine load, enabling the ECM to adjust the C-4 system accordingly. High vacuum indicates (to the ECM) that the engine is either idling or cruising and low vacuum indicates engine operation at or near wide-open throttle.
Throttle Position Sensor (TPS)
The TPS monitors the degree of throttle opening, allowing the ECM to alter the air/fuel mixture according to engine operating conditions. The TPS is mounted inside the carburetor.
Carburetor Model E4ME
This model of carburetor is used only with computer emissions systems. The E4ME plays a key role in the control of exhaust emissions, in that it is a "solenoid controlled" carburetor. The solenoid used in the E4ME is termed a Mixture Control (MC) solenoid. After the ECM "digests" information from the various monitoring devices, it signals the MC solenoid to alter the air/fuel mixture to compensate for the operating conditions of the engine. The MC solenoid is actually a combination fuel flow valve and air bleed valve, as the metering of fuel or air can be controlled independently. The solenoid acts to supplement the idle and main metering systems of the carburetor.